UAPremExecflyer and sk3 give you good advice (as always!). As you can see from the T3 Arrivals map supplied by sk3 there are NO bancomats (ATM machines) inside the secured are of Terminal 3, but there are TWO change-your-money stations.

And that's done for a reason. Don't be tempted to exchange money - or draft an account - here. These are the WORST exchange rates you can get. Wait until you clear the secured area and you are in the public area of T3.

Besides the bancomats in the non-secured area... there are still one or two change-your-money machines. Again, stay away from these and use a bancomat. Page to the bottom of this article and you'll see some photos of the bancomats in the T3 non-secured area. You might also read this posting on bancomats in Italy. They're really not much different than what you're used to perhaps.

Sometimes the machines aren't broken - but during, before, and after a major holiday... they're OUT OF MONEY! So it's best to have a few euro (as suggested by UAPremExecflyer) to get you to your first stop!